Peak temperatures of more than 2000° C. can occur during the combustion of the fuel-air mixture in the combustion chamber of an internal combustion engine. A means of cooling is required in order to prevent a thermal overload of the materials used for cylinder head, valves, spark plugs, injection valves, cylinders, pistons, piston rings, gaskets, etc. Forced circulation cooling by means of a cooling fluid has become widely established for this purpose. In such a system cylinder and cylinder head are implemented as double-walled. The interspace is filled with a cooling fluid and embodied in such a way that a coolant circuit is produced. A mixture of water, antifreezing agent and inhibitors specific to the particular situation is used as the cooling fluid.
Such conventional cooling systems usually include a coolant pump that is driven by the internal combustion engine either directly or indirectly by way of a moving traction mechanism, e.g. a fan belt, and an expansion material thermostat. The coolant pump therefore operates as a function of the rotational speed of the engine and is configured in such a way that an adequate flow of coolant is made available in every operating state of the internal combustion engine. The coolant temperature is regulated in order to maintain a coolant temperature, and hence also an internal combustion engine temperature, that remains constant within narrow limits. Toward that end a temperature-dependent expansion material controller is provided which actuates a valve that allows an increasing flow of coolant to stream past the radiator if the coolant temperature decreases. The expansion material controller and valve form a structural unit and are generally referred to as a radiator thermostat.
Starting from the cold operating state of the internal combustion engine, the radiator thermostat is initially closed and the circulation of coolant takes place exclusively in a bypass circuit of the internal combustion engine. This is also referred to as the “small cooling circuit”. At or above a specific coolant temperature the radiator thermostat opens and the flow of coolant is conducted to the radiator, is cooled down there owing to the air stream and/or the radiator fan, and is conducted back again to the internal combustion engine. This is also referred to as the “large cooling circuit”.
DE 102 26 928 A1 discloses a method for operating a liquid-cooled internal combustion engine in which the coolant is circulated as necessary by means of a coolant pump within a closed coolant circuit. As a function of a variable characterizing the temperature of the internal combustion engine, the coolant volume flow is switched over by means of an actuating element from a first coolant circuit connecting a coolant inlet and a coolant outlet of the internal combustion engine to a second coolant circuit containing a radiator of the internal combustion engine. At the coolant outlet of the internal combustion engine the coolant volume flow can be split as a function of said variable into a first coolant volume flow in the first coolant circuit and into a second coolant volume flow into a bypass containing at least one oil coolant heat exchanger. This means that after a cold start of the internal combustion engine has been detected the actuating element can be controlled in such a way that the coolant volume flow is channeled exclusively via the bypass containing the oil coolant heat exchanger, thus leading to rapid heating of the lubricants such as engine oil and/or transmission oil and/or hydraulic oil.
A particularly rapid warmup of the internal combustion engine, and in consequence thereof also of the lubricants, is achieved if initially, starting from cold start conditions of the internal combustion engine, no circulation of the coolant takes place, resulting in very rapid heating of the relatively small coolant volume contained in the cooling jacket of the internal combustion engine. This can be achieved, for example, by means of a suitable coolant mixing valve or, in the case of a coolant pump driven mechanically by the internal combustion engine, by provision of a switchable coupling. In cooling systems having an electrically driven coolant pump the cooling circuit can be interrupted in a simple manner by switching off the electric motor of the coolant pump. Since in this case the coolant no longer circulates, it is also referred to as a “standing coolant”.
Toward that end it is proposed in DE 102 26 928 A1 to use an electrically driven coolant pump which is switched off at this operating point of the internal combustion engine. As a result of the thus achieved minimization of the warmup time and reduced friction due to the lower oil viscosity at higher temperatures, fuel consumption is lowered and more favorable emission characteristics are to be observed into the bargain.
The problem that arises with such an approach resides in the fact that coolant temperature sensors are usually arranged outside of the internal combustion engine, mostly in a line at the coolant outlet of the cylinder head, and consequently no longer supply reliable signals concerning the thermal operating state of the internal combustion engine itself, in particular concerning the temperature prevailing in the cylinder head. In order to obtain an accurate value for the temperature of the internal combustion engine nonetheless, even when the coolant pump is deactivated, recourse is made at least in the warmup phase of the internal combustion engine to the signal of a temperature sensor arranged at or in the cylinder head of the internal combustion engine.
Since the operation or, as the case may be, non-operation of the coolant pump therefore has an effect both on the warmup behavior of the internal combustion engine on the one hand, and on the emission characteristics, in particular at the time of a cold start, on the other, the pump must be monitored in order to verify that it is operating correctly. A defective or deactivated coolant pump can lead to unacceptable overheating of the internal combustion engine, while a coolant pump that is always active at the time of a cold start of the internal combustion engine can lead to increased pollutant emissions.